Histologically, the thyroid gland appears as formed by the juxtaposition of many cell blocks which we give the name of thyroid vesicles. Each vesicle is formed and thyroid:
* the center is occupied by a body more or less voluminous gummy substance, yellowish, devoid of any cell. This substance is called colloid;
* each cluster of colloid is surrounded by a single layer of polyhedral epithelial cells. These are cells that develop the colloid to have in reserve in the center of vesicles;
* between thyroid vesicles are very rich capillary network. The appearance of thyroid cells and the amount of colloid contained in the vesicles vary depending on the degree of activity of the gland.


Thyroid Hormones
Thyroid hormones are synthesized and stored within the colloid. It consists of a protein substance whose chemical composition is still incompletely known, thyroglobulin, resulting from the combination of thyroid hormones or their precursors with a globulin.
Thyroid hormones are released from thyroglobulin. These hormones are:
the di-iodo thyronine or T2-, tri-iodo-thyronine or T3, and tetra-iodo-thyronine or thyroxine or T4. These hormones, linked to the thyroglobulin molecule that serves as reserves are released and discharged into the blood where they are attached to transport proteins. Thyroxine alone represents about 75% of circulating thyroid hormones, other hormones, the remaining 25%. These hormones are, in contrast to thyroglobulin, chemically known perfectly. The share taken by the physiological effects of each hormone compounds is uneven: the two most active are the T3 and T4, with a leading role in T3.


The essential fact to know is very rich in iodine of thyroid hormones. Iodine is determined by the avidity with which the gland used for the synthesis of hormones in the laying on thyroglobulin. Also the presence of iodine is indispensable to the activity of the thyroid, any deficiency in iodine determining thyroid hypofunction and goitre. Recent work has uncovered a new thyroid hormone, totally different, thyrocalcitonin. It is a natural protein hormone acting solely on calcium metabolism.
The thyroid gland is endowed with multiple functions that are the consequence of the action it exerts on the metabolism in the cell floor, action we will look first.

Metabolic Actions
The action exerted on the thyroid cell metabolism is the common denominator of all functions of the gland. Generally, thyroid active combustion processes at the cellular level, it is somewhat faster turn the “power station” human. It works like this:
* Energy released by the cell: removal of the gland decreases cell activity and consequently the energy released by the cells, administration of thyroid extracts on the contrary increases the cellular metabolism. Cellular metabolism can be measured easily by basal metabolism: it is significantly lowered (-30% to -45%) after removal of the gland, there are increased cases of thyroid hyperfunction;
* on the metabolism of carbohydrates, lipids, proteins which it accelerates the utilization by the cells of the body, use less if thyroid hypofunction;
* This increase in general metabolism causes an increase in respiratory exchange, an increase of circulating blood volume and cardiac output resulting clinically by palpitations and hot flashes in case of thyroid hyperfunction; hypothroidie causes the opposite phenomena;
* This metabolic action generally plays an important role in regulating body temperature: thyroid fight against cuts in temperature by increasing the production of body heat;
* finally recall that the thyroid plays an important role in the metabolism of iodine. Most of the iodine the body is determined by the gland in the colloid and is used in the development of thyroid hormones.
This action of thyroid stimulation exerted on the cellular activity explains the fundamental role of this gland during the growth period of intense cellular activity. The removal of the thyroid gland in patients undergoing stops the growth of the latter and causes dwarfism thyroid, administration of thyroid extracts corrects problems caused by the removal of the thyroid and this with the results even better than the replacement therapy was begun earlier;
Inadequate thyroid occurring in very young children causes growth retardation in height and weight associated with a considerable lack of intellectual and sexual development (cretinism).

Shares tissue
The thyroid affects the different tissues of the body:
- on the growth plates which prepares the maturation and ossification;
- the female genitalia: the presence of the thyroid is essential for the development of young and female genitalia in particular the onset of puberty;
- on the Annexes to the skin (hair and nails) and teeth and it promotes the growth of hair, nails, the emergence and growth of teeth;
- cell of the higher nervous system: it facilitates the operation of the latter and thereby affects the intellectual development and mental condition. Moreover, the functioning of thyroid disorders cause a stir constantly intellectual and temperamental at Rights: cretinism in children, mental slowing in adults with hypothyroidism, irritability and excitability in hyperthyroidism.

The Thyrocalcitonin
It acts on calcium metabolism: it leads to hypocalcemia primarily by direct action on the system by inhibiting bone resorption. This braking bone catabolism is very important and it significantly reduces the amount of calcium released from the bone. Thyrocalcitonin also determines the hypercalciuria in addition to the above mechanism to achieve hypocalcemia.
The act also thyrocalcitonin on the metabolism of phosphorus, intimately linked to calcium: it leads to hypophosphatemia (decreased blood levels of phosphorus) by the same mechanism and also increasing the urinary excretion of phosphorus.
This hormone has a role antagonistic to that of parathyroid hormone (which is hypercalcemia) in maintaining serum calcium, vital necessity.

Regulating thyroid
Obeys the thyroid hormone secreted by the anterior pituitary gland: thyroid stimulating hormone, or TSH The removal of the pituitary reduced by 90% the activity of the thyroid gland. The pituitary itself is under the control of the hypothalamus, which secretes a hormone which stimulates the secretion of pituitary thyroid stimulating hormone: the hormone is the TRF secretion, and therefore is determined by thyroid hormone levels circulating thyroid: increased secretion in cases of lower rates of circulating thyroid hormones, and vice versa. In human cases of hyperfunctioning thyroid (Graves’ disease) was found abnormal thyroid activator, the LATS, whose activity is very close to that of TSH.
The secretion of thyrocalcitonin is totally independent of pituitary control and depends only on the rate of calcium, increases in the latter leading to increased hormone secretion, and any decrease braking secretion.

exploration and examination
Several tests can be studied in clinical abnormalities of thyroid function:
* The measurement of basal metabolism: increased in hyperthyroidism, decreased in hypothyroidism.
* The determination of blood cholesterol: decreased in hyperthyroidism, increased in hypothyroidism.
* The yarrow reflexogramme: lying in hypothyroidism, hyperthyroidism shortcut.
* The study of thyroid fixation of radioiodine. It allows to draw curves and determination to make maps of the gland. The iodine binding is more rapid than the gland is hyper functioning.
* The blood levels of circulating iodine and thyroid hormones: their rate is even higher than it is a thyroid hyperfunction.